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Full text of "Text Book Of Mechanical Engineering"

Advantages of Compounding.                   621

horse power per minute, will represent 4the indicated horse power
of the engine, the formula|becoming

T  j-                            /         •   x     /L0 N

Indicated horse power (per mm.) = --------

33,000
*
Taking the high pressure cylinder in Fig. 624, the addition of

the pressures on the left diagram, 73, 103, &c. = 679*5, anc^ ^le
mean pressure = 67*95. The right diagram similarly has a mean
pressure of 59*5, the final mean pressure becoming 67*95 + 59'5
-r- 2 = 6372. Area of cylinder is 10 x. 10 x 22 -=-7 =* 314* 16,
stroke is 3 feet, and number per minute 63 x 2 — 126. We
have then:

T      '_  .              ...         6372 x 3 x 314*16 x 126

LH. P. in H. P. cylinder = -£J------------±-2.------------ = 229*3.

33,000                    --------

In the intermediate cylinder, mean pressure on the left is 23-9
and that on the right 22, the final mean being 22*95. Area of
piston = 855*3; stroke and*revolutions as before.

___._.              ...         22*95 x 3 x 855*3 x 126

I. H. P. in i. P. cylinder = —™------------H-*---------= 224-44.

J                             33,000                    —122-

Mean pressuFes on left and right respectively in low pressure
cylinder are 9*5 and 7*65, and the|mean of these is 8*57. Then,

T TT _   .              ...         8*57 x 3 x 2290*22 x 126

I. H.P. in L. P. cylinder = —il—j2-------2——-------- = 224*41

}                             33,000                   ----ail-

Advantages of Single, Double, and Triple Stage
Expansion.—The advantage {of expanding steam in a single
cylinder, instead of using full (pressure to the end of the stroke,
was demonstrated by Watt in 1782, and can be understood from
Fig. 625. E G is the stroke, and FN that portion during which full
steam is used, the rest of the stroke, N G, being completed by the
pressure of the expanding steam. From what we know of the
work diagram, SBcu will shew work performed by the 'full'
stearn, without condensation, UCKT that by expansion without
condensation, and HSTJ that produced by the use of a condenser
Not only then do we obtain additional work by condensing, but
we are also enabled with the assistance of high pressure to
introduce an earlier cut off and higher rate of expansion: thus
using a less weight of live steam.